6/29/2015 ExaminingGPSData(GPSandGIS)Part4

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ExaminingGPSData(GPSandGIS)Part4

AnAerialPhotoThatCanBeUsedasaMap:TheDigitalOrthophoto

Adigitalorthophotoimageisanaerialphotograph,indigitalform,withaveryusefulspecialproperty:Ithasbeencalibratedsothatitmaybeusedasamapthecoordinatesofthemaparepreciselyspecified.Asyoumayknow,mostaerialphotographs,evenwhentakenwiththecamerapointedverticallydownward,haveconsiderabledistortion,duetoseveralfactorssuchasuneventerrainandlensdistortion.Thus,ifyoumeasurealengthonaconventionalphotographthatcorrespondstoadistanceontheground,anidenticallengthmeasuredelsewhereonthephotographusuallywillnotcorrespondtothesamedistanceontheground.Saidconcisely,astandardaerialphotographdoesnothaveaconsistentscale.Inadigitalorthophotoimage,however,objectsshownonthephotographatgroundlevelarerelatedbyapointsonobjectsabovegroundlevel(forexample,ontheroofsofbuildconsistentscale,eveniftheterrainisdramaticallyuneven.(Thelocationsofings)arenot,however,shownintheircorrectlocations.)

Digitalorthophotosmaycomeinmanyforms.Theonewelookathereiscalleda"tiff"(whichstandsfor"taggedimagefileformat."PathfinderOfficehasthecapabilityofdisplayingsuchfilesasbackgroundfiles.Theextensionoftiffnamesisusually"tif."Ifatifffileiskeyedtoageographicallocationitmaybecalleda"geotiff"geotiffscontaintheparameterstoanchortheimagetotheproperpointsontheearthssurface.Ifatiffimageiskeyedtoageographiclocationbutcontainsnogeographicparametersthenitmustbeaccompaniedbyaworldfileworldfilesusuallyendintheextension"tfw."Pathfinderofficerequiresthatatiffbeanongeotiffandthatitbeaccompaniedbyaworldfile.18

InthedirectoryByeByeSAarethefollowingfiles:

No_sa_ncr_doq.tifataggedimagefilethespecialkindofdigitizedaerialphotographthatcanbeusedasamap,showingthehighwaytheautomobilewastravelingatthetimeoftransition.ThetermDOQinthenamestandsfordigitalorthoquadranglequadrangleisjustanothertermforan"almostrectangular"map.19

No_sa_ncr_doq.tfwthe"worldfile"forNo_sa_ncr_doq.tifcontainingnumericparametersthattellPathfinderOfficewheretoplacetheimageNo_sa_ncr_doq.tif.

NoSA_Moving.SSFasubsetofafiletakeninamovingautomobilecoveringtheminutesoftransitionwhenSAwasdiscontinued.

{__}Open__:\GPS2GIS\ByeByeSA\NoSA_Moving.SSF.Representthefilewithamediumyellowline.SettheTimeZonetoEasternDaylight.Bringupthetimelineandnotethetimespanofthefile.IfyoulookatPositionPropertiesyouwillnotethatanumberoffixeshavebeendeleted.Theideawastocapturejustthetimesurroundingthetransition.

{_}PreparethemapoftheGPStracksothatthedigitalorthophotowillbeproperlypositioned:Beforetheaerialimagecanbedisplayedthegeographicparametersofthemapmustbeproperlyset.TheDOQQQQ20(standsfordigitalorthoquarterquarterquarterquadranglesinceitisonesixtyfourthofastandardUSGS7.5minutequadrangle)isintheU.S.StatePlane1983coordinatesystem,KentuckyNorthZone(1601).ThedatumisNAD1983(Conus).Thecoordinateunits,altitudeunits,anddistanceunitsareSurveyFeet.UsingtheUnitsandCoordinateSystemchoicesundertheOptionsmenu,makesuretheseparametersaresetupcorrectly.

{__}Bringupadigitalorthophotoimageasabackgroundtothefilesyouhaveselected:FromtheFilemenuselectBackground.IntheLoadBackgroundFileswindowremoveanyfilesthatshowupandthenclickAdd.Thewindowthatappears(AddBackgroundFiles)mightormightnotrepresentthefolder__:\GPS2GIS\ByeByeSA.(Ifitdoesnt,navigatetothatfolder.)Selectthefileno_sa_ncr_doq.tifandclickonOpen.Readandokayareminderifitshowsup.BackintheLoadBackgroundFileswindowreviewtheSystem,Zone,andDatumspecifications.Itprobablyappearsthatalliswell,butitmaynotbe.ClickonChangetoexamineandcorrectparameterssotheymatchtheonesgiveninthepreviousstep.BeparticularlycarefulregardingCoordinateUnits.In

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6/29/2015 ExaminingGPSData(GPSandGIS)Part4

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KentuckytheyareinSurveyFeet!OkaytheCoordinateSystemwindow.OkaytheLoadBackgroundFileswindow.AnimageshouldappearintheMapwindow.

{__}MakethemainPathfinderOfficewindowoccupythefullscreen.MaketheMapwindowwideasinFigure38,soyoucanhavePositionPropertiesandTimeLineupalso.ZoomuponthepartoftheyellowtrackthatcrossestheDOQ.LookattheGPStracksthatrepresentthepartofthetriponthedividedhighwaythattookplaceduringthetransitionfrom"SAon"to"SAoff."Firstthetrack,comingfromthewest,iswayofftheroad,thenyouseesomejaggedspikesthatoccurredprobablyduetothetransition,andfinallythetraceisatleastintherightofwayofthehighway.WithPositionpropertiesyoucanascertainthatthetransitionwascompletedatabout13secondsafter12:01AMon2May2000.

PROJECT3EYourData

NowthatyouhaveagoodideaofhowtousePathfinderOfficetodisplayandanalyzedatatakenwithaGPSreceiver.

{__}Beforebeginningthecomputerwork:

Unlessyourinstructortellsyouotherwise,youmustusethemachineontowhichyouloadedyourdata

Figure38.Watchselectiveavailabilitygoaway.

RecallthenameoftheProjecttowhichyoutransferredyourdata.ItisprobablyDATA_yis(where"yis"indicatesyourinitials).Thefolderinwhichyouwillworkis__:\GPS2GIS\DATA_yis.

Bringwithyouthefullname(s)ofthefile(s)youcollected.AlsohavewithyouthevaluesyoucalculatedbyhandinProject1C.

Bringthedetailedmap(s)ofthearea(s)whereyoutookthedatasoyoucancomparewhatappearsonthescreenwiththefeaturesshownonthemap.

{__}Ifnecessary,startthePathfinderOfficesoftware.SelectProjectsintheFilemenu.MakesurethattheProjectnameis"DATA_yis."ClickontheOpenFoldericonontheProjecttoolbar.Clickthe"Details"buttontoseethedatesonthefilesandfolders.Reviewthefilesthataretheretobesurethatyouareintherightfolder.YoushouldseeaBackup,Base,andExportfolderaswellasyourfiles.Select,withasingleclick,thefirstofthefilesyoutook.Lookattheinformationaboutthatfileatthebottomofthewindow.Closethewindowwithoutopeningthefile.

{__}ExaminethemapyouusedinProject2Atodeterminethecorrectunits,coordinatesystem,anddatumtouse.Setupthesoftwarewiththeseparameters.

{_}Therepresentationnowonthescreenshouldbeofthepoints(fixes)youcollected,connectedinthetimeorderinwhichtheywereloggedbythereceiver.

Thefollowingstepsassumethatyoutookpointdata.

{__}YousettheGeoExplorerroveroptionsbeforethatsessionaccordingto:

6/29/2015 ExaminingGPSData(GPSandGIS)Part4

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Coordinates~Deg&Minutes(anddecimalfractionalpartsthereof)

Datum:(settoyourmap)

Units~Custom~Distance:(settoyourmap)

Units~Custom~AltitudeUnits:(settoyourmap)

Units~Custom~AltitudeReference:Geoid(MSL)

Date&Time~SetLocalTime:(adjusttolocaltime)

SetthePathfinderOfficeoptionstotheequivalents.

{__}SelectPositionProperties.Clickonafixthatistowardthemiddleoftheclusterofpoints.Readitslocationatthebottomofthescreenandwriteitdownhere.

Latitude_____________

Longitude____________

Altitude_

Isitaboutwhereyouexpectedittobe?CompareitscoordinatevaluesagainsttheaveragedvaluesinPROJECT1Cthatyouwrotedownatthattime.

WecouldmakeaPoint_genericfile(averageallthefixesintoasinglepoint)aswedidinProject3B,butletslookatanotherwayofgettingtheaverage.ThedatacollectedbytheGPSreceiveranddownloadedintothemachinetaketheformofrecords,whicharenumberedstringsoftext.AgivenGPSfilewillhavequiteafewrecordsatleastoneforeveryfixtaken,andusuallyalotmore.YoucanlookattheserecordswithaTrimbleUtilitycalledtheSSFRecordEditor.WhentheRecordEditorisinvokeditautomaticallyaveragesthecoordinatesofallthefixesandappendsadditionalstatisticalrecordstothefileattheend.InthestepbelowyouwillopenthefileintheRecordEditor,lookbrieflyatafewrecords,andthenreadthecoordinatesofthe"average"point.

{__}Gointothefilemenuandpick"Close"toclosethecurrentfile.(TheRecordEditorwillnotcalculatestatisticsonafileifthefileisopen.)Youhavealreadysetuptheproperparametersofdatum,coordinatesystem,andunits.IntheUtilitiesmenuselectOther,thenSSFRecordEditor.Theappropriatewindowwillcomeup.UnderFileinthatwindowclickOpen.Youshouldthenbeabletoselectthefileyouhavebeenworkingwithfromthewindowthatshowsup.ClickOpen.MaketheSSFRecordEditorwindowoccupythefullscreen.

Theresultingwindowisdividedintotwoparts.(Youcanvarythepositionofthedividinglinebetweenthembydraggingitwiththemouse"Split"intheViewmenualsoworks.)Inthetoppartaretherecordsofthefile,beginningwithaheaderrecordwhosethreeletterdesignationisHDR.TheHDRrecordishighlighted.Inthelowerpartofthewindowaredetailsinamorereadableformatrelatingtothathighlightedrecord.Also,somerecords,likeHDR,containinformationnotshownontheupperpartofthewindow.Thisinformationappearsinthebottomportionofthewindow.Makesurethatthebottompartofthewindowislargeenoughtoseeallthetextlines.PleaseseeFigure39.

MostSSFfileshaveseveraltypesofrecords.Shortlyyouwillseethattowardthemiddleofthefileyouwillfindposition(POS)recordsthattellyouthetimeandpositionofeachseparatefix.Attheendofthefilearestatisticsrecords(STS).

{__}LookattheinformationintheHDRrecord.Thenusethedownarrowkeyand/orPageDowntolookatrecordsfurtherdowninthefile.Finally,dragtheverticalslidertothebottomsothattheSTSrecordsthatwereaddedtotheendofthefileshowup.Usethe"uparrow"keytoselectthe"LatitudeStat"record.Writethecoordinatesofthatandthefollowingrecordshere.

MeanLatitude

MeanLongitude:

MeanAltitude:21

{_}FromtheFilemenuexittheSSFRecordEditor.

{__}Someofthequestionsbelowhaveseveralblanks.AssumingyoutookdataforbothPROJECT1BandPROJECT2Ainthesamelocation,recordthecalculationsyoumadeinPROJECT1C.

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Figure39.FindingstatisticsfromtheRecordEditor.

{__}Setupthedatum,coordinatesystem,andunitstocorrespondtothoseofthemapyouusedinPROJECT2B(whereyoumovedtheantennaalongapath).

{__}BringinthefileofPROJECT2Banddisplaythepathyoutookwhilemovingtheantenna.Nowqueryvariouspointsonthedisplayandcomparetheircoordinateswiththoseonthemap.Doesthedisplaycorrespondtotheactualpathyoutook?

{__}AddthefilethatcontainsthefixesyoucollectedinPROJECT2A.Arethetwositesinthecorrectrelativeposition?Ifnot,youmaybeusingdifferentprojections,differentdatums,ordifferentcoordinatesystems.

Nextpost:DifferentialCorrection(GPSandGIS)Part1

Previouspost:ExaminingGPSData(GPSandGIS)Part3

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